In accordance with the rapid advance of semiconductor elements and semiconductor circuits such as thyristors, transistors and integrated circuits and their applications, the use of semiconductor elements and semiconductor circuits in instrumentation, control and communication equipment and power equipment is now widespread, and these equipment make rapid progress toward miniaturization and higher performance. On the other hand, despite such advances, these equipment and parts used therein are not regarded satisfactory in withstand voltage, surge rating and noise immunity. It is then a very important task to protect such equipment and parts from abnormal surge and noise or to establish a stable circuit voltage. For solving these problems, there is a demand for the development of a voltage-dependent nonlinear resistor material which has substantial voltage-dependent nonlinearity, great discharge withstand current rating, and improved life property and is inexpensive.
Used for such purposes are varistors containing silicon carbide (SIC), selenium (Se), silicon (Si), ZnO or the like as a major component. Among others, the varistors based on ZnO are generally characterized by a low clamping voltage and a great voltage-dependent nonlinearity index. These varistors are then suitable for protection again overvoltage of equipment composed of elements having a low overcurrent rating such as semiconductor elements and have been widely utilized as a substitute for SiC-based varistors.
By the way, such ZnO-based voltage-dependent nonlinear resistors are generally prepared, like voltage-dependent nonlinear resistors based on other materials, by firing a compact of a voltage-dependent nonlinear resistor-forming source powder containing ZnO as a major component according to a firing process including a heating step, a high temperature holding step and a cooling step. In the prior art, the entire firing process was carried out in an atmosphere having a constant oxygen partial pressure (typically ambient air), but no varistors thus obtained had a nonlinearity index .alpha. in excess of 100, with .alpha. being normally about 50.
JP-A 106102/1984 proposes a method for preparing a ZnO-based varistor wherein the oxygen partial pressure of the firing atmosphere used in the firing process is switched from below to above 2.times.10.sup.-1 atm (air's oxygen partial pressure) in a time region from a point in a later stage of the high-temperature holding step to a point immediately after transition to the cooling step, for the purpose of providing an increased .alpha. value.